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Short-range networks based on wireless mesh networking architectures have evolved to enable a power- and cost-efficient means to enable the remote management of non-computer devices. Self-organizing mesh network architectures make a whole new crop of wireless machine-to-machine applications possible, including motion-detection sensors used on the battlefield; thermometers gauging the temperature of food products or pharmaceuticals in transit; and medical devices monitoring patient vital signs.
Wireless sensor networks can be designed in a variety of ways to address different priorities and make the appropriate technology trade-offs based on the requirements of the application. There are a number of inter-related design variables associated with wireless sensor networking. These variables are shown in Table 1.
Click here for Table 1
In this article, we'll look at the variables associated with developing mesh networking products. We'll then examine the various mesh networking topologies available.
All wireless mesh networking systems share a set of common requirements. These include:
- Small module form factor— A very small form factor for the network modules is needed so that endpoints can fit inside or attach easily to an existing device.
- Low power consumption — To support long-term operation, the power consumption of the radio link must be minimized so that devices can be powered by compact, lightweight batteries such as a coin cell battery for long periods of time.
- Range — It is more power efficient to emit low strength RF singles to travel a short distance and be relayed a number of times than to transmit higher strength signals for longer range. Repeaters form a network using a protocol that supports multi-hop routing so that data packets can be relayed from one repeater to another when the mobile RF terminal is far away from the base station.
- Minimal human intervention — The network protocol allows the sensor network to initialize itself in a highly ad hoc, self-organizing manner. This allows patients wearing sensors to move around, and enables doctors and nurses to move in and out of the network with their data-gathering gateways.
- Bi-directional communication — Communication between the gateway and sensor is bi-directional to enable the base station to transmit signals to adjust certain operating parameters in addition to receiving signals transmitting sensor data.
- Reliability — While data reliability is always important, it becomes a critical requirement for many applications, for example in medical monitoring and requires a mechanism for persistent routing of data through the network.
- Responsiveness — Topology discover and re-discovery must be efficient, especially for applications where sensor nodes are mobile, such as in mobile machines or equipment or for wearable sensors.
- Scalability — The network must support the number of nodes required immediately and must also be able to support future growth without causing exponential growth of overhead.
A robust networking protocol is needed to meet the above requirements as well as those of a particular mesh networking design. The networking protocol provides support for the network's topology and manages the routing of data through the network. In order for the application to benefit from the promises of wireless sensor networking, the underlying protocol must support all of these basic requirements.
Topologies
The critical first step to developing any wireless sensor networking application is to profile the application's requirements in terms of the variables shown in Table 1. This exercise should determine which variable (e.g. data rate, mobility, etc.) will be the leading design driver in the overall design and operation of the network and will help determine what design trade-offs may need to be made. This will also help you identify the appropriate topology for your application.
There are several architectures that can be used to implement wireless sensor network applications, including star, mesh, and star-mesh hybrid. Each topology presents its own set of challenges, advantages, and disadvantages. In order to select the appropriate topology for your wireless sensor network application, it is important to understand the tradeoffs each topology requires.
Below we'll look at each topology in detail. But, in order to understand the topologies, you need to be familiar with the components of a wireless sensor network. These components include:
Endpoints — Integrate with sensors and actuators to capture the sensor data. Endpoints are also referred to as reduced-function devices (RFD).
- Routers — Extend network area coverage, route around obstacles, and provide backup routes in case of network congestion or device failure. In some cases, routers can also act as endpoints. Routers are also referred to as full-function devices (FFD)
- Gateways — Aggregate the data from the network, interface to the host, LAN, or the Internet, and act as a portal to monitor performance and configure network parameters.
- System Software The system software provides the networking protocol to enable the self-configuring, self-healing ad hoc network.
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